1. Field of the Invention
The present invention relates to a Wavelength Division Multiplexing (WDM) submarine optical repeater system and particularly to a submarine optical gain equalizer installed on a submarine optical transmission line, and the submarine optical transmission line and its installation method.
2. Related Background Art
In the WDM optical transmission system, to transmit multi-wavelength optical signals over a long distance, it is necessary to install light amplifiers at predetermined intervals on an optical transmission line so as to amplify signal light. The light amplifiers, however, have gain characteristics slightly different among wavelengths, which causes level differences between optical signals having longer wavelengths and shorter wavelengths. These level differences are accumulated in each light amplifier. Accordingly, some lines are disabled from being used for communications in long-distance optical transmission lines. Therefore, in a shore optical transmission system over a transmission distance of several hundreds of kilometers or so, light source power of signal light having different wavelengths has been previously adjusted based on the characteristics of the light amplifiers, for example.
In a submarine optical communication system, however, with multi-wavelength light having an optical transmission line exceeding 10,000 km, for example, a method of adjusting the light source power of the signal light does not function appropriately in some cases. In the submarine optical communication system including such a long-distance optical transmission line, there has been no proposal of any specific configuration of gain equalization for adjusting respective wavelength levels. It is also possible theoretically to achieve an equipment for detecting respective wavelength levels automatically and for adjusting the levels. It is presumed, however, that this kind of the equipment is extremely complicated and requires high cost.
It is an object of the present invention to provide a submarine optical gain equalizer for compensating efficiently for gain differences between wavelengths of multi-wavelength optical signals transmitted on an actual submarine optical transmission line at low cost, the submarine optical transmission line and its installation method.
The submarine optical gain equalizer of the present invention comprises a pressure housing and an optical gain equalizer installed in the pressure housing having input and output ports formed in the outside of the pressure housing. In the pressure housing, one optical gain equalizer can be installed. In addition, in the pressure housing, one or more through-fibers can be installed in the pressure housing. The optical gain equalizer attenuates levels of optical signals having specific wavelengths by predetermined amounts. On the submarine optical transmission line of the present invention, one or more submarine optical gain equalizers in the above are installed. An installation method of the submarine optical transmission line comprises detecting respective wavelength levels of multi-wavelength optical signals in respective optical fibers composing the submarine optical transmission line and connecting submarine optical gain equalizers for compensating for the predetermined level differences between the detected wavelengths in the detected positions of the optical fibers. This invention provides an efficient and low-cost compensation for the level differences in multi-wavelength light transmitted through the long-distance submarine optical transmission line.